Loose Tappet Guides: The most common repair the XK cylinder
heads require is due to one or more loose tappet guides. The tappet guide is a
steel sleeve, machined to press-fit into the head as a guide for the valve
tappet. It is a tight fit and under normal circumstances does not move. The
symptom is a clatter, usually evident in the exhaust cam cover area, although
the symptom can appear on the intake side as well. This occurs due to the
engine being overheated, which then causes the aluminum of the head to expand
away from the guide allowing the guide to back out of the head along with the
tappet, bringing the guide into contact with the cam lobe.(Tappet starting to come up below far left picture)

Removal of the cam covers is
necessary to confirm the problem. If the owner is lucky and did not run the
engine too long while the symptom occurred, he might not need to have any
tappets replaced. At this point, the factory method can be used to lock down
the guides without removing the cams. However, if the guides appear damaged or
broken the cams will have to be removed in order to inspect the affected
guides (Severe case withDamaged Valve Guide above far right). Also, since the valve covers are off this would be a good time to
adjust the valves.

Inspection of the guides with the
cams removed will reveal only certain scenarios. The lobe might mark one or
more tappets, leaving them essentially undamaged. Sometimes the guide will be
raised slightly and look higher than the others, and if so, it should be tapped
back down into place before being locked down. Impact by the lobe might wear
away some of the rim of the guide, leaving a jagged edge. This does not
necessarily cause a problem, so long as the tappet moves smoothly in the guide
and the edge of the guide can be utilized for one of the locking methods
describe below. If the guide is broken
then another will have to be installed in its place.

Used guides are easier to install
than new ones. This is because the new ones are oversize on the outside and
undersize on the inside. This requires special machining of the head and then
of the tappet guide once installed for proper clearance. Not all machine shops
are capable of doing this job properly so a good used guide is usually a much
less expensive choice.

Jaguar has a special lockdown kit
for holding the tappet guides down (Center Picture above). The plates are set over the guide edges and
do not rest on the aluminum of the head below the guides. Boltholes for
securing the plates to the head are aligned and marked through the holes in the
plates. Holes are drilled to the size necessary and self-tapping screws are
used. We at Coventry West drill and tap threaded holes above the guide in an
area both where the aluminum is substantial and where oiling is not inhibited.
We then install cap screws to secure the guide.(Picture above second fromright) When the guide is clattering it
is basically "floating" out of the head's casting, so the head of the
screw is sufficient to hold down the guide. We continue to use our method as we
developed it years before Jaguar came out with a fix for the problem. We
naturally believe our method is better but both methods work equally well. Locking
the tappet guides down is recommend as a matter of preventative maintenance. If
you decide to adjust your valves or if your cam covers are off for cleaning or
polishing, then this would be a perfect time to lock down your tappet guides.

Cracks Due to Overheating: Before work on
a head begins, the first problem area to check is cracks. At the outset, it
must be stated that the XK head rarely cracks and it usually happens from a
severe overheat. Cracks are difficult to perceive with the naked eye, so we seal
the water passages of the head and pressurize the water jacket with air while
the head is submersed in water. If the head has a crack it shows up as fine
bubbles escaping from the affected area.

Cracks appear in typical places. One area often seen
is a hairline branching out from the expansion plug (commonly referred to as
freeze plugs) between cylinders 3 and 4 and going into the spark plug hole. (See picture above Far Left) The early heads have a threaded plug here instead of
an expansion type. These early heads seem less prone to cracking. Cracks are
also found between a valve seat and its adjoining spark plug hole.(See Picture Second from Left Above ) Almost any
crack can be repaired but the cost of repair must be compared against the value
of the head to justify the job. Early heads are usually numbered and matched to
a car where as the later heads are not. It may be worthwhile to repair an older
head in order to maintain originality. Also, the highly visible cracks, such as
the ones on the top of the head, are the most difficult to repair if appearance
is an issue.

Corrosion Issues: Corrosion is a problem
on old heads and must be addressed before the head is rebuilt. It is found
anywhere there is water, which includes the steam holes and water passages
between the head and the block, the passages from the head and the intake
manifold, and on rare occasions a head can develop a porous spot. We've seen
porosity form in the intake passages. This can be found only by pressure
testing as described above. Welding aluminum into the affected area is the only
long term proper repair for corrosion. Almost all XK heads require welding on
the sealing surface to the block, especially the late heads with the small steam
holes. Welding up the corrosion minimizes the amount of material that must be
removed during resurfacing to obtain a flat surface. (See Picture Below Left)

Corrosion in an aluminum head is directly related to
the level of maintenance done to the engine during its lifetime. Most people
don't realize that antifreeze has more than the antifreeze and antiboil
functions. It also has anticorrosion properties, especially necessary for
aluminum. Therefore, proper maintenance of an engine entails flushing the
cooling system and installing fresh antifreeze regularly. The newer, longer
life antifreeze will help minimize corrosion for a longer period. Keep in mind,
however, that “Lifetime Antifreeze” really means "for the life of the
antifreeze", not "for the duration of your life". It still must be changed on a regular basis.

Outer Surface Issues: These heads have
been in existence for a long time, and have been worked on by many people.
Needless to say, they need a great deal of attention in order to be made
presentable again.

General Head
Preparation:

Cleaning: All heads are
steam cleaned and bead blasted. The large cylinder head stud passages are
exposed to engine coolant and usually coated with corrosion. These are
rebored. All stud holes are chased and
blown out with compressed air.

Broken Studs:
Any broken studs must be removed and damage to the holes repaired. (See Picture Below Right)

Damaged threads: All damaged
stud holes are drilled oversize and steel threads, commonly referred to as Heli
Coils, are installed.

Spark Plug Hole Damage:
These, too, must be drilled oversize and replaced by a steel insert. Spark plug
hole repairs can be avoided by using anti-seize compound on the threads when
the plugs are changed.

Tool Marks and Other Abuses:
Heads can be difficult to remove. Anyone who has tried to remove one knows
this, whether it's the hobbyist working at home or the professional. They will
resort to any and all techniques, no matter how destructive, to get the head
off. Hammer and prybar marks must be smoothed and filled in with filler such as
JBWeld before the head is painted. (See Picture Second From Left below)

Scrapers quite
often mar the cam cover surface. I do not like the idea of grinding this
surface down with hand tools because hand tools leave the surface rounded. I
prefer to fill in the marks and then paint the surface lightly. The paint is
just for appearance, and the home mechanic would not need to apply paint here,
but filling in the scrape marks will give a better sealing surface for the cam
cover gasket. Do not worry about paint on gasket surfaces, we've found that it
has no negative effect on the ability of a gasket to seal.

Another area that
requires attention is gouging of the washer
surface for the acorn nuts that hold the cylinder head down. This gouging can
only be caused by someone in the past tightening the head down without flat
washers installed. It is astonishing that any mechanic would do this, but we
see it all the time. (See Picture Below)

Please note that
if you are doing your own work, the XK engine used different lengths of
cylinder head studs, depending on the position and type of hoist brackets
(assuming it came with hoist brackets). If you have engine hoist brackets, you
will notice that a thinner washer is used on them. The positions of the hoist
brackets vary, too, depending on the model of Jaguar. Be sure to make note of
the bracket and washer placement when disassembling your engine.

Cylinder head build process: After cleaning and
preparation, the head is sent to the machine shop for surfacing, valve guide
replacement, and valve seat grinding.

Head warpage and its effects: It is common knowledge that if a head is
overheated, it is probably warped and therefore must be resurfaced. However,
what is not common knowledge is that head warpage not only affects the sealing
surface; it can affect the valve adjustment, too. When a head warps, the whole
head warps. When a head is surfaced at the machine shop, it straightens only
the surface that mates to the block. The rest of the head can still be slightly
warped in the cam area. Once the head installed on the block, the cam sprockets
are in place, and the chain tightened, the cam can pull down too far and the
valves for number one cylinder can become too tight. We compensate for this
when adjusting the valves on the bench and by using new cam bearings on the
journals closest to the front of the engine. We recommend checking the valve
adjustment after both cams are installed. Be sure your number one cylinder (the
one at the front) is at TDC on the compression stroke (the distributor rotor
pointing to the five o'clock position) when you install the cams, and don't let
the cams 'snap' ahead allowing the valves to hit the number 6 or number 1
piston during installation!

Upgrades: There are upgrades that can be
done to the heads that offer more performance and also improve reliability and
longevity.

Steam Holes: Later XJ saloon
heads came with steam holes between the combustion chambers, which mate to
holes in the block. The early 4.2L engines did not have the holes at all. If an early head is installed on later block,
the engine can overheat. This is because the heat around the cylinder walls
will create steam pockets which have nowhere to go and will force the coolant
out of the system. This will further reduce cooling capacity causing further overheating. We drill steam holes into the early heads.
This also allows an early head to be installed in place of a later head without
risk. A later head can be installed on an older block, with no adverse effect,
as the holes will be redundant.

Enlarging the Valves:
Carbureted heads had smaller intake valves than did the fuel injected heads.
Most machine shops can upgrade these to accommodate the larger valves,
providing a nice increase in performance.

Valve Guide Replacement: We
replace the guides on the pre 1968 heads with new bronze style guides. The
later guides have a groove in them that holds the valve guide seal in place.
They also have a retaining ring to locate the guide in the head. The earlier
guides were held in place through an interference fit only and seals were not
used.

Camshaft
Selection: Around 1968, Jaguar changed the camshaft profiles and the
adjustment specification. These camshafts produce a lot less valve clatter and
do not hurt performance. They can be put in to the earlier heads requiring
nothing more than a valve adjustment at a different specification. These cams
do not have an oil hole on the bottom of each lobe as the earlier cam do. This
does not seem to hurt anything as the tappet valley is filled with oil with
bleed off from the cam bearings and this is more than adequate for lubricating
the tappets.

Multi
Angle Valve Seats: This type of
valve grind is commonly referred to as a “three angle” valve grind or a
“performance” valve grind. This type of grind involves cutting the seat in at
least three different angles. The valve is cut at 45 degrees and the main
contact seat also is 45 degrees. The other two angles are on the seat and are
usually 30 and 60. Some can even have more angles. This lets the air flow over
the seat flow easier and as such lets in and out more air. This in turn helps
the engine produce more power. A side benefit is that this type of job usually
last longer than a single angle grind. The contact surface of the seat is
consistent all the way around so the valve cools evenly. If one side had a
wider contact surface, the valve would cool more on that side and would tend to
warp causing loss of compression.